1. Introduction
This invention relates to removal of dissolved contaminants from photoresist solutions containing acid-labile materials and more particularly, to the use of novel ion exchange procedures for the purification of photoresist solutions.
2. Description of the Prior Art
Ultra pure liquids free of particulate, ionic, and organic contamination are required for many industrial purposes such as for the manufacture of pharmaceuticals and integrated circuits. For example, in the manufacture of high resolution integrated circuits, it is known that many processing liquids come into contact with a bare wafer or a resist coated surface. These include photoresists and treatment chemicals such as organic liquids and aqueous solutions which contain acids, bases, oxidants, and other proprietary ingredients. At least 15 to 50 liquids of various compositions are used to clean wafers, prime surfaces, deposit resists or other polymers, and to develop, rinse, etch and strip the resist. It is known that these solutions may be a source of contamination of the integrated circuit wafer that may interfere with its performance. Thus, the reduction or removal of insoluble and soluble contaminants from processing fluids used for the production of integrated circuits before or during use is basic insurance for prevention of damage to the integrated circuit.
Photoresist coating compositions are used extensively in integrated circuit manufacture. Such compositions typically comprise a light-sensitive component and a polymer binder dissolved in a solvent. Typical photoresist compositions are disclosed in U.S. Pat. Nos. 5,178,986; 5,212,046; 5,216,111; and 5,238,776, each incorporated herein by reference for disclosure of photoresist compositions, processing, and use.
It is known that photoresist coating compositions contain particulate and ionic contaminants. For example, it is known that solid gels or insolubles form in photoresists due to dark reactions. In addition, soluble impurities such as moisture, silicone oils, plasticizers, and metal ions may be present from the manufacture of the resist components. In Class 100 clean rooms, airborne particulate counts amount to 3 particles per liter of density of 2. By comparison, liquids contain about 100,000 particles per liter. A particle count of 100,000 per liter seems high, but if translated into a solid of 0.6 .mu. in size, this is equivalent to 10 parts per million parts of solution (ppm). A level of 10 ppm amounts to the deposition of 10 mg. per liter. Since liquids are heavily contaminated compared to clean room air, effective contaminant removal is essential to the manufacture of such devices.
Ultrafiltration of resist liquids has progressed and manufacturers of resist now supply resist materials filtered through 0.04 .mu.M diameter absolute filters. However, methods for removal of particulates from treatment solutions are not effective for removal of dissolved contaminants from solution such as organic impurities and ionic species. These contaminants can be at least as damaging to an integrated circuit as particulate contamination.
Efforts to remove dissolved cationic and anionic contaminants from treatment solutions used to manufacture integrated circuits are known in the art. For example, one such method is disclosed in International Publication No. WO 93/12152, incorporated herein by reference, which is directed to removing metal ions such as sodium and iron from novolak resins during manufacture. The process comprises cation exchange treatment whereby a cation exchange resin is first washed with a mineral acid solution to reduce the level of total sodium and iron ions within the exchange resin to preferably less than 100 ppb, passing a formaldehyde reactant through the so treated cation exchange resin to decrease the sodium and iron ion content to less than 40 ppb, passing a phenolic compound through the cation exchange resin to decrease its sodium and iron ion content to less than 30 ppb, and then condensing the so treated phenolic compound with formaldehyde in the presence of an acid catalyst to form the resin.
A method for removal of dissolved ionic metals and non-metals from a photoresist is disclosed in published Japanese Patent Application No. 1228560, published September 12, 1989, incorporated herein by reference. In accordance with the procedures of this patent, a photosensitive resin is passed through a mixed bed of a strong cation exchange resin and an anion exchange resin to simultaneously remove cation and anionic species from the photoresist solution.
The procedures of the prior art may be used to treat many, but not all photoresist compositions. In particular, the process of the prior art is unsuitable for treatment of photoresists that operate through an acid catalyzed reaction mechanism. Such photoresists generate a positive or negative image through a mechanism whereby a light-sensitive, photoacid generator (PAG), when struck by light, photolytically decomposes generating an acid that either causes a polymerization reaction in light-struck areas to produce a negative image, or causes cleavage of a pendant acid labile group to solubilize a polymer constituent of the photoresist in light-struck areas to produce a positive image. Examples of negative-acting, acid catalyzed photoresists are disclosed in EPO 0 232 972 and U.S. Pat. Nos. 5,210,000; 5,212,046; 5,229,254; and 5,234,791, each incorporated herein by reference for the disclosure of negative-acting acid catalyzed photoresist compositions and their use. Examples of positive-acting acid catalyzed photoresists are disclosed in U.S. Pat. Nos. 4,491,628; 5,229,256; and 5,258,257, each incorporated herein by reference for its teaching of positive photoacid generating photoresist compositions and their use.
In practice, it has been found that when prior art procedures are used to remove solution-soluble ionic contaminants from acid catalyzed photoresists of the type disclosed in the above-cited patents, strong acid groups of a cation exchange resin react with the PAG component of the photoresist formulation thus decreasing the PAG concentration in solution to a lower and unpredictable level. The decrease in PAG content results in batch-to-batch concentration variation in the resist. This in turn results in batch-to-batch variation in photoresist properties, especially photospeed. Batch-to-batch variations in properties are unacceptable to electronic device manufacturers.
Another problem with the process of the prior art involves the use of a strong anion exchange resin. Though the anion exchange resin removes contaminants from solution, it does so by an exchange reaction. The exchange reaction results in introduction of impurities into the photoresist that are exchanged with the anions of the anion exchange resin. Though the impurities introduced into the photoresist are more tolerable than the contaminants removed, any impurity in a photoresist is considered undesirable.